Electrostatic machine



Nov. 28, 1939. I w. D. COOLIDGE El AL I 2,181,724

I ELECTROSTATIC MACHINE Filed Sept. 21, 1958 Inventqrs: Wil Ham D.Coo|idge William K. Kearsley.

Their Attorney.

Patented Nov. 28, 1939 UNITED STATES EIECIROSTATIO MACHINE William D.Coolidge and William K. Ke'nrsley,

Schenectady, N. Y., assignors to General Electric Company, a corporationof New York Application September 21, 1938, Serial No. 231,086

4 Claims. (01. 171-329) The present invention relates to improvements inelectrostatic apparatus of the type which employs a continuousinsulating belt as a means for conveying electric charges from acharging device to a high potential charge accumulator.

It is an object of the invention to increase the amount of charge whichcan be carried by a belt of given size and to decrease the likelihood ofsparkover from the various parts of the apparatus. This object isaccomplished by maintaining the opposed sides of the belt in directcontact in the region between the charging device and the accumulator.

The features which we desire to protect herein are pointed out in theappended claims. The

invention itself may best be understood by reference to the followingdescription taken in connection with the drawing in which Fig. 1illustrates an electrostatic machine suitably embodying the invention,and Fig. '2 is an enlarged view showing in detail the belt-chargingelectrodes employed.

Referring particularly to Fig. 1, the electrostatic machine showncomprises an endless belt 25 ill, for example of paper, which issupported in the form of an elongoted loop by means of rollers H and I2.The rollers are mounted insuitable bearings and the belt is-driven bymeans of a motor l4 connected to the roller II. The belt is 30 given anelectric charge by means of charging devices provided adjacent to therollers, and the charges are conveyed to a spherical accumulating deviceI 6 which is provided intermediate the ends of the belt. An insulatingstandard I! sup- 35 ports the accumulator and insulates it from ground.

For charging the belt we may employ any Under these conditions, assumingthat a poten-.

tial on the order of several thousand volts is 50 employed between theelectrodes l8 and, I9, a brush discharge occurs from the electrode I.The negative elements of this discharge tend to flow to the electrode IEbut are intercepted by the upper surface of the belt III, which thusreceives 55 a negative charge. The rounded contour of the electrode l9prevents a similar positive discharge from taking place from thatelectrode.

As a result of the motion of the belt 10 the negative charges referredto are'carried toward w and into the interior space of the accumulatorl6 negative charges borne by the incoming belt por:

through an opening 24 provided therein and are caused to impart acorresponding charge to the accumulator. Additional charging 'electrodes25 and 26, identical in flmction with the electrodes i8 and I9, causeadditional negative charges 5 to be brought to the accumulator by thelower portion of the belt moving inwardly from the right.

The negative charges on the belt are transferred to the accumulator bymeans now to be 10 described. .In order to take these charges from theincoming portion of the belt there is provided a comb-like electrode 30similar to the electrode l8 previously described. This'is positionedadjacent to the surface of the belt and receives charges from it bymeans of a. brush discharge. These charges are communicated to acylindrical electrode 3| which is directly conuected to the electrode 3.and which is also positioned closely adjacent to the belt surface. The

electrode 3| is further arranged to cooperate with another comb-likeelectrode 32 positioned adjacent to the bottom half of the belt loop andY connected to the accumulator l6. By virtue of the negative charge ofthe electrode 3|, a high gradient is established between this electrodeand the electrode 32'. This gradient is in such direction as to cause abrush discharge which carries positive charges from the electrode 32toward the electrode 3|. Such charges are,.however, intercepted by thesurface of the lower belt half and are thus carried out of theaccumulator. This withdrawal of positive charges from the accumulatorobviously increases its negative potential with respect 'to ground. Anegative po- 35 tential of one million volts or more may readily bebuilt up in this way. I y

In connection with the right-hand side of the belt loop and within theaccumulator l6 there is provided a second electrode system identical in40 operation with thesystem which includes the electrodes 30, 3| and 32.This second system includes an electrode 34 for picking up negativecharges from the incoming lower portion of the belt and an electrodecombination 35, 36 for causing positive charges to be transferred fromthe accumulator to the outgoing surface of the upper belt half. Ingeneral, the positive charges borne away from the accumulator by theoutgoing,

portions of the belt are substantially equal to the tions.

The accumulator Ii may be connected to the negative terminal of a loaddevice, such, for example, as an X-ray tube, and may be causedto 55 Ienergize the device. The success in this connection of an electrostaticmachine of the type under consideration depends primarily on the amountof current which it can supply. This, in turn, depends partly upon therate at which electric 6o charges can be conveyed to and irom theaccumulator. For open belts this latter quantity is limited mainly bysparking from the collector to the oppositely charged half of the beltand thence to ground and by the tendency of charges imparted to the beltto escape to surrounding objects when their value is increased unduly.Such escape is apparently a result of ionization of the surrounding airby a corona or brush discharge proceeding from the belt.

We have found that the total current carried by the belt may besubstantially increased by operating the two sides of the belt loop indirect contact or in substantially direct contact in the regions betweenthe charging means and the accumulator. While we do not wish to be boundto any particular theoretical explanation of this result, we believethat it is due to the fact that the close adjacency of substantiallyequal and opposite charge on the upper and lower belt halves tends todecrease the voltage stress which can exist between the belt as a wholeand any external object. .Stated in another way, it appears that withthe arrangement specified the negative charges are bound to the belt bythe close ad glaclency of the positive charges'on the other belt Itmight appear that placing the belt halves in contact as specified wouldincrease the tendency to direct spark-over between them. That this isnot the case is due to the fact that with a given charge density thevoltage gradient between the belt-halves is independent of theirspacing. This result follows directly from1the known relationship thatthe voltage between two spaced surfaces is proportional to the quotientof the charge on the surfaces and the capacitance between them. Sincethe capacitance of two elements is inversely proportional to thedistance between them, it will be seen that for a fixed charge,decreasing the spacing of the belt-halves necessarily decreases theirpotential difierence. Consequently, the potential between the belt sidesmay actually be very small when they are in direct contact, so that thetendency to sparkover between them is not increased by operating them inthis condition.

In order to maintain the belts in the desired relationship duringoperation there are provided rollers 31, 38, 39 and 40 which serve toforce the two belt' sides into direct contact in the regions between thecharging devices and the accumulator IS. A fifth roller ll maintains thebelt halves in spaced relation within the accumulator so as tofacilitate the removal of charges.

While we have stated that the belt-halves are in direct contact it maybe that when in motion they are separated by a thin film of air.Obviously no great difference in operation should be expected even ifthis is the case.

With the arrangement shown the current which may be conveyed from thecharging devices to the accumulator is substantially increased. Theclose adjacency of'the positively and negatively charged belt halvesappears to lessen materially the likelihood of a discharge occurringalong the surface of the belt. Furthermore due to the decreased coronathe total belt length which must be provided between the accumulator andthe charging electrode system may be lessened without danger ofspark-over between the accumulator and the grounded portion of theapparatus,

. The latter effect becomes especially important in an arrangement inwhich the apparatus as a whole is to be immersed in a gaseous dielectricother than air, In a dielectric such as vaporous carbon tetrachloride ora fluorine substitution product of carbon-tetrachloride (such as CClzFz)the over-all dimensions of the apparatus may be substantially reduced bydecreasing the belt length as specified. Consequently a relativelycompact apparatus can be built by utilizing the principles of theinvention.

While we have described our invention in connection with an arrangementin which the belt extends completely through the accumulator this is byno means essential. Ihe invention is equally applicable in connectionwith other arrangements such as that in which only one end of the beltprojects into the accumulator. Numerous further modifications may bemade by those skilled in the art without departing from the invention,and we aim in the appended claims to cover all such variations as comewithin the true spirit and scope of the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An electrostatic machine of the type which employs a chargeaccumulator, a charging device, and a continuous belt for carryingcharges between the charging device and the accumulator, wherein theopposed sides of the belt operate in substantially direct contact in theregion between the charging device and the accumulator.

2. An electrostatic machine including a continuous belt, means forcharging the belt, a charge accumulator for collecting charges from thebelt, means for driving the belt, and means for causing the opposedsides of the belt to operate substantially in direct contact in theregion between the charging device and the accumulator.

3. An electrostatic machine including a continuous belt, means forsupporting the belt in a closed loop, means for driving the belt, meansassociated with the belt at one portion thereof for" charging the belt,a hollow accumulator spaced from the charging means and so arranged thatits interior is traversed by a portion of the belt, means within theaccumulator for receiving charges from the belt, and means formaintaining the opposed sides of the belt loop in mutual contact in theregion between the charging'means and the accumulator and formaintaining the sides of the belt in spaced relationship within theaccumulator.

ii An electrostatic machine including a continuous belt, meanssupporting the belt in a.

closed loop, means for drivingthe belt, means WILLIAM D. COOLIDGE.WILLIAM K. KEARSLEY.

